1. Field of the Invention
The present invention relates generally to processes and systems for separating a bio-distillate fraction from a bio-oil. More specifically, the invention relates to the production of a renewable distillate including at least in part the bio-distillate fraction. An embodiment of the present invention also relates to processes for upgrading a bio-oil by use of a diluent and/or a recycle stream from the upgrading process to reduce fouling in upgrading equipment, such as a hydrodeoxygenation unit.
2. Description of the Related Art
With the rising costs and environmental concerns associated with fossil fuels, renewable energy sources have become increasingly important. The development of renewable fuel sources provides a means for reducing the dependence on fossil fuels. Accordingly, many different areas of renewable fuel research are currently being explored and developed.
With its low cost and wide availability, biomass has increasingly been emphasized as an ideal feedstock in renewable fuel research. Consequently, many different conversion processes have been developed that use biomass as a feedstock to produce useful biofuels and/or specialty chemicals. Existing biomass conversion processes include, for example, combustion, gasification, slow pyrolysis, fast pyrolysis, liquefaction, and enzymatic conversion. One of the useful products that may be derived from the aforementioned biomass conversion processes is a liquid product commonly referred to as “bio-oil.” Bio-oil may be processed into transportation fuels, hydrocarbon chemicals, and/or specialty chemicals.
Despite recent advancements in biomass conversion processes, many of the existing biomass conversion processes produce low-quality bio-oils containing high amounts of oxygen which are difficult, if not impossible, to separate into various fractions. These bio-oils require extensive secondary upgrading in order to be utilized as transportation fuels and/or as fuel additives due to the high amounts of oxygen present in the bio-oil. Furthermore, these transportation fuels and/or fuel additives derived from bio-oil vary in quality depending on the original oxygen content of the bio-oil.
Accordingly, there is a need for an improved process and system for separating bio-oil into various fractions such as bio-distillate, and using such fractions as blend components of renewable fuels.
Further, although bio-oil obtained from biomass catalytic pyrolysis is more stable than thermal pyrolysis oils, it is still generally less stable than petroleum derived hydrocarbons. This instability is associated with bimolecular reactions, such as condensation and/or polymerization reactions, of oxygen-containing compounds. At the typical elevated reactor temperatures of hydrotreating units (such as 300-500° C.), these instability reactions become more pronounced resulting in heavier compounds which create deposits in both the hydrotreating feed pre-heater and in the hydrotreater unit. In order to minimize such fouling in commercial operations, the preheater and hydrotreater temperatures are kept as low as possible while still sufficiently high to provide effective hydrodeoxygenation. However, even with such temperature control there is still significant fouling requiring frequent and costly unit shutdowns for deposit removal, hydrotreating catalyst regeneration, and/or hydrotreating catalyst replacement. Accordingly, there remains a need for an improved process for upgrading bio-oil which reduces fouling of the upgrading equipment.